Ideas on why I can’t conceive??

Hi. So I’ve been ttc for 4 months now. Using opk’s bbt’s and preseed. Well my partner and I both believe that we might have something wrong with our bodies. Our first we conceived within the first month (Aug 2019) then got pregnant (June 2020 and I miscarried) now I’ve been ttc again and have been timing everything right. (I’m not stressed or overthinking etc) but I have yet to get pregnant (my period are sorta irregular 26-31 and 4 days long) but before my partner for years (5yrs) I was unable to get pregnant then all of a sudden bam pregnant like I said first try with him. So I don’t understand what’s going on. We have no idea If he has any issues (eg low sperm count etc) or if I have any (eg PCOS, infertility etc) but what could be causing us not to conceive minus stress and trying?